1. Field of the Invention
This invention generally relates to a positioning device and, more particularly, to a method for detecting pressure on a resistive touch sensing element and an electronic device using the same.
2. Description of the Related Art
FIG. 1 shows a conventional 4-wire touch screen 9 that is composed of two flexible transparent resistive plates, an X resistive plate 91 and a Y resistive plate 92, separated by a gap, wherein the X resistive plate 91 and the Y resistive plate 92 are plane resistances and do not contact each other in natural state. On the X resistive plate 91, the resistance only changes with the position along an X direction. The X resistive plate 91 includes two electrodes X+, X− disposed along the left and right sides of the X resistive plate 91 and a transparent resistive layer formed on the upper surface of the X resistive plate 91. On the Y resistive plate 92, the resistance only changes with the position along a Y direction. The Y resistive plate 92 includes two electrodes Y+, Y− disposed along the front and back sides of the Y resistive plate 92 and a transparent resistive layer formed on the lower surface of the Y resistive plate 92, wherein the X direction and the Y direction are two directions perpendicular to each other.
FIG. 2 shows a cross-sectional view of the touch screen 9 being pressed. When a touch point P on the Y resistive plate 92 is pressed by a force F, a distance between the X resistive plate 91 and the Y resistive plate 92 at the touch point P decreases such that a contact resistance Rtouch changes to a definite value from an infinite value. When the force F is larger, the contact resistance Rtouch becomes smaller. In the touch screen 9, a contact pressure of the force F can be determined through calculating the contact resistance Rtouch.
Please refer to FIGS. 3a to 3c, they show a conventional method for calculating the resistance value of the contact resistance Rtouch. The contact resistance Rtouch may be obtained from equation (1),Rtouch=(Rx—plate)×[(X/2n)×(Z 2/Z1)−1]  (1)
In equation (1), RX—plate is a surface resistance of the X resistive plate 91; n is the resolution of an analog-to-digital converter (ADC) connected to a voltage detection point; X is a position of the touch point P in the X direction and it may be obtained according to the digital voltage outputted by the ADC shown in FIG. 3a; Z1 is an input of the Y− electrode and it may be obtained according to the digital voltage outputted by the ADC shown in FIG. 3b; and Z2 is an input of the X+ electrode and it may be obtained according to the digital voltage outputted by the ADC shown in FIG. 3c. 
However, the pressure detection method through calculating the resistance value of the contact resistance Rtouch has at least the following problems.                1. The resistance value of the contact resistance Rtouch is obtained through calculation, but the X in equation (1) is an uncertainty factor itself.        2. The resistance value of the contact resistance Rtouch is not equal to the pressure, and thus it only can be served as a reference value rather than a basis for representing actual pressure.        3. The range of the contact resistance Rtouch is very large and thus it is difficult to define a critical point according to the calculated resistance value such that the accuracy of identification is limited.        
Accordingly, it is necessary to provide a method for detecting contact pressure on a touch sensing element with high accuracy so as to reduce the possibility of misidentification of a touch point.